A multiple-input multiple-output (MIMO) wireless communication system may obtain diversity and array gains by using beam forming (BF) and receive signal combination, or by using precoding and receive signal combination. Typically, a wireless communication system using BF or precoding may be generally expressed as y=HVs+n, where y is a received signal vector, H is a channel matrix, V is a precoding matrix, s is a transmitted symbol vector, and n is measurement noise. Generally, optimal precoding requires that channel state information (CSI) is fully known to a transmitter, and a commonly used method is that a user equipment (UE) performs quantization on instantaneous CSI and feeds back the quantized instantaneous CSI to a node B (NodeB). CSI fed back by an existing Long Term Evolution Release 8 (LTE R8) system includes a rank indicator (RI), a precoding matrix indicator (PMI), a channel quality indicator (CQI), and the like, where the RI and the PMI respectively indicate the number of used layers and a precoding matrix. Generally, a set of used precoding matrices is called a codebook, and each precoding matrix in the codebook is called a code word. A codebook of LTE R8 is mainly designed for single user multiple-input multiple-output (SU-MIMO), where a precoding matrix or a code word is restrained by 8 phase shift keying (8PSK), and precision of spatial quantization is limited. For a transmission manner in which precision of spatial quantization is sensitive, such as multiple user multiple-input multiple-output (MU-MIMO), performance of the transmission manner is severely limited by the codebook of LTE R8. To meet a higher system requirement, a 3rd Generation Partnership Project (3GPP) LTE system needs to further enhance performance of MU-MIMO. In addition, a coordinated multiple-points (CoMP) transmission technology is introduced (currently, CoMP is based on a single-cell feedback). The foregoing two technologies both have a higher requirement on feedback performance.
In the prior art, a single codebook is used to feed back RIs and PMIs. By using four antennas as an example, a correspondence between an RI, a PMI, and each code word in the codebook is shown in the following Table 1:
TABLE 1RIPMIun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/{square root over (2)}W0{124}/{square root over (3)}W0{1234}/21u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/22u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/23u3 = [1 j 1 −j]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/24u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/25u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/26u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/27u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/28u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/29u9 = [1 −j −1 −j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
Wn{s} indicates a matrix that includes a column set {s} of a matrix Wn=I−2ununH/unHun, where I is a 4×4 identity matrix, and un is provided by the foregoing table. In an existing LTE R8 system, there are two PMI feedback manners: one manner is that one PMI is fed back for entire system bandwidth; and the other manner is that, the system is divided into multiple bandwidth parts (BP) and each BP includes multiple sub-bands, and PMIs are fed back for different sub-bands. The former manner is generally called broadband PMI, and the latter manner is generally called sub-band PMI.
It can be learned from thorough investigation on the foregoing Table 1 that, in a precoding matrix obtained based on Wn=I−2ununH/unHun, in the prior art, a phase of each element is subject to an 8PSK restraint; that is, a phase difference between different elements is a multiple of π/4, and it is difficult to describe a smaller spatial granularity difference, such as a phase difference of π/16 or π/8 between antenna ports. Therefore, feedback precision of CSI is reduced, and it is difficult to meet a feedback precision requirement of a technology such as MU-MIMO or CoMP transmission.